As a method for producing a potassium hydroxide aqueous solution, a method is known wherein a potassium hydroxide aqueous solution is produced by electrolyzing a potassium chloride aqueous solution by an ion exchange membrane method using a cation exchange membrane as a diaphragm.
In the ion exchange membrane method, in an electrolyzer equipped with a cathode and an anode, a cation exchange membrane of potassium salt form as a diaphragm is installed to divide inside of the electrolyzer into a cathode compartment on the cathode side and an anode compartment on the anode side; and a potassium chloride aqueous solution is supplied to the anode compartment, and a potassium hydroxide aqueous solution is supplied to the cathode compartment, to electrolyze the potassium chloride aqueous solution while maintaining the concentration of the aqueous potassium hydroxide solution discharged from the cathode chamber to be at a predetermined concentration (e.g. 32 mass %).
However, in the ion-exchange membrane method using the cation exchange membrane of potassium salt form, the membrane is likely to be swollen and expanded during electrolysis of the potassium chloride aqueous solution. In the electrolyzer, the cation exchange membrane of potassium salt form is fixed along the peripheral edge to the frame, and therefore, if the membrane is swollen and expanded, wrinkles will be formed in the membrane. If wrinkles are formed in the cation exchange membrane of potassium salt form, there will be such a problem that the membrane is damaged by the contact between the membrane and the electrode, or the electrolysis voltage rises as bubbles are accumulated in the wrinkles.
As a method of suppressing formation of wrinkles in the cation exchange membrane of potassium salt form, a method has been proposed wherein just prior to installing the membrane in the electrolyzer, the membrane is permitted to absorb water and swell to bring elongation of the membrane to be from 3 to 9%, whereupon the periphery of the membrane swelled by absorption of water is fixed to the frame to install the membrane in the electrolyzer (Patent Document 1).
However, such a method has a problem that it requires a work load and site for the operation for swelling by absorption of water immediately before mounting the membrane in the electrolyzer.